1. Field of the Invention
The present invention relates to a composite material having high mechanical strength, toughness, and clarity, and also to a process for producing the same. The composite material is composed of a resin composition containing a polyamide and a layered silicate constituting a clay mineral which are bonded to each other through ionic bond and are uniformly mixed with each other.
2. Description of the Related Art
Various attempts have been made so far to incorporate an organic polymeric material with an inorganic material such as calcium carbonate, clay mineral, and mica for the improvement of its mechanical properties. As the result of such attempts, the present inventors developed a composite material composed of a resin containing a polyamide and a layered silicate having a layer thickness of 7-12 .ANG. uniformly dispersed therein, with the polymer chain of said polyamide being partly connected to said silicate through ionic bond. (See Japanese Patent Laid-open No. 74957/1987 (which corresponds to U.S. Pat. No. 4,739,007).) This composite material has a high elastic modulus and heat resistance because of its unique structure; that is, silicate having an extremely high aspect ratio are uniformly dispersed in and connected to a polyamide resin through ionic bond. This composite material, however, is subject to brittle fracture even at room temperature under a comparatively small load. Therefore, it is not necessarily satisfactory in mechanical strength.
In the meantime, the crystalline polyamide resin as a typical engineering plastics exemplified by nylon-6 and nylon-66 finds use as automotive parts and electric and electronic parts on account of its high melting point and high rigidity. A disadvantage of the crystalline polyamide resin is that it is opaque on account of its crystalline structure. This leads to a problem arising from the fact that automotive parts such as reservoir tanks, radiator tanks, and fuel tanks made of polyamide resin make the liquid level invisible from outside and the electronic parts such as connectors made of polyamide resin prevent the detection of conductor breakage therein.
Unlike the crystalline polyamide resin, the amorphous polyamide resin having the aromatic skeleton structure is transparent. An example of the amorphous polyamide resin is "Trogamid" made by Dynamit Nobel Co., Ltd. Unfortunately, it is extremely expensive and cannot be a substitute for aliphatic nylons such as nylon-6 and nylon-66. Moreover, the aliphatic nylon extremely decreases in strength and heat resistance when it is made amorphous. Under these circumstances, there has been a demand for a polyamide resin which has high clarity without decrease in crystallinity.
The above-mentioned polyamide-silicate composite material developed by the present inventors has a higher clarity than a polyamide resin; but the clarity is not so high as to solve the above-mentioned problems involved in automotive parts and electronic parts.
In order to eliminate the above-mentioned disadvantages, the present inventors carried out a series of researches, which led to the present invention.